1. Field of the Invention
The present invention relates to a mobile communication system including a scheduler which causes a base station to select a mobile communication terminal as a communication destination in response to communication requests from mobile communication terminals, and a mobile communication terminal of the system.
2. Description of the Related Art
As one mobile communication system, the 1xEV-DO system based upon a technical standard document “C.S0024 cdma2000 High Rate Packet Data Air Interface Specifications” by a standardizing organization “3GPP2” is known. In the EV-DO system, a mobile communication terminal measures the reception quality of a downstream channel by a pilot signal transmitted from a base station. On the basis of this measurement result, the mobile communication terminal determines a data modulation method, and notifies the base station of the determined modulation method. The base station transmits data to the mobile communication terminal as a request source by using the notified data modulation method. In this system, as long as the reception quality of a mobile station is high, data can be transmitted by using a high data transmission rate although the error resistance is low. On the other hand, if the reception quality is low, data can be transmitted by a modulation method having a low speed but a high error resistance.
The cdma2000 1xEV-DO system uses TDMA (Time Division Multiple Access) as a downstream data transmission communication method in the direction from a base station to a mobile communication terminal. In this access method, time is divided into units of 1/600 sec (to be referred to as slots hereinafter), and mobile communication terminals requesting communication are allocated in one-to-one correspondence with these slots. In this manner, each mobile communication terminal is temporarily allowed to exclusively possess one slot. By the use of this method, the base station can transmit data with the maximum transmission power to each mobile communication terminal. In this way, data communication can be performed at the highest communication rate between the base station and each mobile communication terminal.
Conventionally, a scheduling algorithm (to be referred to as a scheduler hereinafter) to be described below, for example, is used to allocate mobile communication terminals to the slots. That is, each mobile communication terminal measures the downstream reception quality of each of a plurality of base stations, and predicts data communication rates usable under the measured reception qualities. The mobile communication terminal then selects a base station having the highest predicted data communication rate, and requests the selected base station to communicate. If communication requests are transmitted from a plurality of mobile communication terminals, a base station selects, from among these mobile communication terminals, a mobile communication terminal which has reported the highest predicted data communication rate. The base station allocates a slot to the selected mobile communication terminal, and transmits packet data. This maximizes the throughput viewed from the base station. However, this also extremely lowers the probability that a mobile communication terminal whose reception quality is not so high is selected. This significantly increases the unfairness of the throughput with respect to receiving environments between mobile communication terminals.
To prevent this, the 1xEV-DO system, for example, uses a PF (Proportional Fairness) scheduler as a scheduler which increases both the throughput viewed from a base station and throughput viewed from a mobile communication terminal. In addition to the downstream reception quality of the mobile communication terminal, the PF scheduler takes account of a data amount which a base station transmitted to each mobile communication terminal in the past, as an index by which the base station selects a mobile communication terminal.
For example, the base station calculates an evaluation function value “DRC/R” for each mobile communication terminal. “DRC” is a predicted data communication rate of a downstream channel, which is notified from a mobile communication terminal to the base station. “R” is a data amount (more specifically, the mean value of the rates of data communications performed in the past) which the base station transmitted to a mobile communication terminal in the past. The base station then selects a mobile communication terminal having the largest calculated evaluation function value “DRC/R”. The use of this PF scheduler raises the probability that a mobile communication terminal having a small past data transmission amount is selected. This makes it possible to alleviate the unfairness of the throughput with respect to receiving environments between mobile communication terminals.
Details of the PF scheduler are described in e.g., IEEE International Conference, VTC 2000 Spring Transactions, A. Japali, R. Padovani, R. Pankaj, “Data throughput of CDMA-HDR a High Efficiently-High Data Rate Personal Communication Wireless System”.
Unfortunately, the existing system using the PF scheduler described above has another problem as follows. That is, a mobile communication terminal unconditionally selects a base station having the highest reception quality, and transmits a communication request. However, the probability of each mobile communication terminal being selected by a base station depends not only on the reception quality or the past data transmission amount, but also on the traffic of the base station. For example, the probability of a mobile communication terminal being selected by a base station having the highest reception quality but high traffic is low. By contrast, if a mobile communication terminal transmits a communication request to a base station having somewhat low reception quality but low traffic, the probability of this mobile communication terminal being selected rises.